Referring to FIG. 1, a typical, known refrigeration circuit for an automobile air conditioner comprises a compressor 1 driven by the automobile engine, a condenser 2, an expansion valve 4 (the first means of expansion) and an evaporator 5. In standard operating fashion, the refrigerant discharged from compressor 1 passes respectively, through condenser 2, expansion valve 4 and evaporator 5, and returns to the inlet port of compressor 1. The refrigerant causes evaporator 5 to absorb surrounding heat and to control air conditioning in the inside of a car. In the refrigeration circuit in FIG. 1, a receiver-dryer 3 may be placed between condenser 2 and expansion valve 4, although it is not always needed. Receiver-dryer 3 functions to absorb water in the refrigerant. Receiver-dryer 3 also may reduce the excess refrigerant, increase the lack of refrigerant and thus improve the efficiency of the refrigeration circuit according to changes of the air conditioning load.
In the above-mentioned refrigeration circuit, the operation of compressor 1 is controlled by an electromagnetic clutch (not shown). Engagement of the electromagnetic clutch is controlled according to a temperature-detector, for example, a thermostat. However, when the electromagnetic clutch is engaged, there is a significant change of torque in compressor 1 which places a drag on the automobile engine, hindering performance and driveability. Additionally, with conventional air conditioning systems, when a car is driven at high speed with a heavy air conditioning load, the temperature of refrigerant discharged from compressor 1 may become too high, which adversely affects both the durability of compressor 1 and the rubber hoses on compressor 1.